EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi2S3 and Bi2O3 semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi-growing film-electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal-film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies (D approximate to 10(-16) cm(2) s(-1)). Solid-state transformation of sulfide to the oxide film is a consequence of OH- ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film-solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal-film interface. (C) 2003 Elsevier B.V. All rights reserved.